As majestic as the Grand Canyon looks from the rim, things get weird when you start to descend one of its cliff faces. As you go down into the canyon, the walls on the other side seem to recede. The farther down you go, the farther away the opposite side moves. The river itself, which was seemingly sandwiched tightly between the cliff walls, seems to stretch away and widen as well.
Halfway down the cliff side, the river is already miles away, and the opposite cliff twice as far beyond that. The few patches of trees you saw along the riverbank have now grown too, becoming wide thickets and copses of wetland vegetation. Strange sounds rise up from below, although you cannot place exactly from where the sounds emanate.
Looking up, the very air seems to shimmer heavily. It has definitely gotten more humid, a gradual but stark change from the thin desert air at the cliff top. The moisture sticks to your skin, and you notice sweat drops forming on your brow. Everything seems to be slightly off-color, sharper than usual, the sun seems brighter, hotter.
After a few more hours hard climbing down, you come to the bottom of the cliff side, and the transformation is complete. The far cliff side is barely visible, just a hazy purple mountain far away, low on the horizon. Before you lays a vast subtropical ecosystem. Thick forests alternate with rock outcroppings, and lagoons, all gently sloping down away from you towards a mighty river valley. The river is wide, as wide as a large lake, and lazy, slowing floating along. Strange shapes can be seen rising occasionally from the water, too small to be seen clearly in the hazy distance.
What adventuring campaign is complete without a Lost World? Who wouldn't want to explore a land full of grunting cavemen, soaring pterydons, hissing lizard men, long-neck grass eaters, and of course, terrifying T-Rexes stalking around threatening to eat everything?
The DMG supplies Lost World random encounter tables [I will update those into this post shortly], and both MMs provide pages of details for the various species of dinosaurs, which would be a pre-requisite bare minimum for DMing the adventures here.
Characters on the lamb: aside from the internal dynamics of the Lost World, I would insert various tie-ins to the surrounding AD&D milieu.
My favorite ideas:
--Powerful psionic master, who can charm the primitive reptiles at will. He uses the giant lizards to enforce his own Lost World kingdom. Perhaps the PCs are led down there searching for one of his kidnap victims: the King's young beautiful daughter, who has been forcibly incorporated into his harem.
--Escape from Dino-Valley! PCs somehow teleported by some trick/trap to the riverside. OUCH. Watch out for those giant Crocs, lads! Can they get out before being eaten? [Even figuring out where they are might be a challenge...]
--T-Rex body parts needed for magical ingredients of some sort, such as its voice box for a Horn of Wall Collapsing, or its jaw bone for Gauntlets of Crushing Grip, its leg muscles for a Potion of Giant Power, its teeth for a Saw of the Titans, its heart/brains/blood for a Scroll of Reptile Control, etc....
--Pterodactyl wings/bones/hide, needed for Wings of Flying
--The paranoid old king seems to have gone completely mad, demanding eggs from giant crocodiles, to raise as his moat monsters. Yeah, he's nutty old fool, but he's offering a king's ransom for those eggs, and rumor says they are plentiful at only one location in the whole state....
--Evil magic user's hidden lair, requires looting for massive treasure and magic haul. Perhaps the resentful cavemen of the surrounding area can be convinced to help in the sacking of his obelisk fortress? Is he even alive, no one has seen him in quite awhile....
Are there any published modules for Lost World settings? Those would be cool, and could presumably be plugged right in.